What Is Soil pH?
pH is a measure of how acidic or alkaline your soil is, on a scale from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. In practical gardening terms, you'll only ever encounter soils between about pH 4 (very acidic peat) and pH 8.5 (chalky alkaline soil).
Why does it matter? pH controls which nutrients are available to plants. Even if your soil is packed with nutrients, the wrong pH locks them up chemically, making them inaccessible to roots. Iron, manganese, and phosphorus become unavailable in very alkaline soil. Calcium, magnesium, and molybdenum become unavailable in very acidic soil.
UK Topsoil pH: What to Expect
Most commercially supplied topsoil in the UK falls between pH 5.5 and 7.5. The exact pH depends on where the soil was sourced:
- Peat and moorland areas (parts of Lancashire, Yorkshire, the Pennines): pH 4.5-6.0, naturally acidic
- Clay lowlands (Midlands, much of South East England): pH 6.5-7.5, neutral to slightly alkaline
- Chalk and limestone regions (Chilterns, Cotswolds, Yorkshire Wolds, South Downs): pH 7.0-8.5, alkaline
- Sandy heathlands (Surrey, Hampshire, parts of Suffolk): pH 5.0-6.5, acidic
BS3882-certified topsoil will have its pH stated on the test certificate, which is one of the advantages of buying certified product.
Testing Your Soil pH
Simple Test Kits
Available from any garden centre for around £5-£10, these use a colour-change indicator solution. You mix a soil sample with the provided solution and compare the colour against a chart. They're accurate to about ±0.5 pH units — good enough for gardening decisions.
Electronic pH Meters
Probe-style meters (£10-£40) give a digital reading. The cheap ones are often unreliable. If buying one, spend at least £20 and calibrate it according to the instructions.
Professional Soil Analysis
For around £20-£40, you can send a soil sample to a laboratory (the RHS offers this service) and get a full analysis including pH, nutrient levels, and organic matter content. Worth doing if you're investing significantly in topsoil or establishing a vegetable garden.
How to Take a Soil Sample
Take samples from several spots across the area (at least 5 for a typical garden), from a depth of 75-150mm. Mix them together in a clean bucket, then test the combined sample. Testing a single spot can be misleading — pH can vary across even a small garden.
What pH Do Plants Need?
Most Garden Plants: pH 6.0-7.0
The majority of common garden plants, lawn grasses, and vegetables grow best in slightly acidic to neutral soil. If your topsoil and existing soil are in this range, you're in good shape for almost everything.
Acid-Loving Plants: pH 4.5-6.0
Rhododendrons, azaleas, camellias, blueberries, heathers, and pieris need acidic conditions. If you're planting these in naturally alkaline soil, you'll need ericaceous (acidic) compost rather than standard topsoil, or grow them in raised beds with acidic growing media.
Alkaline-Tolerant Plants: pH 7.0-8.0
Clematis, lavender, dianthus, brassicas (cabbage family), and many Mediterranean herbs thrive in slightly alkaline conditions. These are easy to accommodate in most UK soils.
Vegetables: pH 6.0-7.0
Most vegetables prefer a slightly acidic to neutral pH. Brassicas (cabbage, broccoli, cauliflower) do better at pH 6.5-7.5 and are less prone to clubroot disease at higher pH. Potatoes prefer pH 5.0-6.0 as higher pH encourages scab disease.
Adjusting Soil pH
Raising pH (Making More Alkaline)
Apply garden lime (calcium carbonate) or dolomite lime (adds magnesium too). Application rates depend on the soil type and how much adjustment you need — typically 200-400g per square metre for a 0.5 unit pH increase on medium soil. Lime works slowly, so apply in autumn for spring effect.
Lowering pH (Making More Acidic)
This is harder and more expensive than raising pH. Options include:
- Sulphur chips: Work slowly (6-12 months), but are the most reliable method. Apply 50-100g per square metre for a 0.5 unit decrease
- Iron sulphate: Faster acting but temporary. Used commonly on lawns
- Organic matter: Composted bark, leaf mould, and pine needles are naturally acidic and gradually lower pH while improving soil quality
When Not to Fight Your pH
If your garden is on chalk with a natural pH of 8.0, you will not realistically maintain acidic soil across the whole garden. Either choose alkaline-tolerant plants or grow acid-lovers in containers with ericaceous compost. The same applies in reverse — trying to alkalinise very acidic peaty soil is a constant battle.
pH and Topsoil Ordering
When ordering topsoil, consider:
- What's your existing soil pH? Test before ordering
- What are you growing? Match the topsoil to the plants' needs
- Will you be mixing old and new soil? The final pH will be somewhere between the two — see our guide on layering topsoil
- Ask the supplier. Reputable suppliers know their product's pH. If they can't tell you, or the topsoil isn't BS3882 certified, consider looking elsewhere
- pH is easier to adjust in raised beds where you control the entire growing medium, than in open ground where the surrounding soil constantly influences pH
The Bottom Line
For most UK garden projects, pH isn't something to worry about excessively. Standard commercially-supplied topsoil falls in the range that suits most plants. Test your existing soil, ask your supplier about their product's pH, and make adjustments only if you're growing something with specific needs or your soil is at an extreme end of the scale.